Nanostructure having metal nanoparticles and method of assembly thereof

ABSTRACT

A nanostructure and method for assembly thereof are disclosed. An exemplary nanostructure includes a photocatalytic nanoparticle; a first tier of metal nanoparticles, each metal nanoparticle of the first tier being linked about the photocatalytic nanoparticle; and a second tier of metal nanoparticles, each metal nanoparticle of the second tier being linked to one of the metal nanoparticles of the first tier and located a distance from the photocatalytic nanoparticle greater than a distance between a metal nanoparticle of the first tier and the photocatalytic nanoparticle.

FIELD OF THE INVENTION

A nanostructure including metal nanoparticles, and a method ofassembling a nanostructure including metal nanoparticles, are disclosed.

BACKGROUND

Gold metal nanoparticles have been used as a pigment to, for example,stain glass. More recently, there has been research into developingmetal nanostructure assemblies, including structures made from noblemetals such as gold and silver. For example, it is known to use anelectromagnetic wave to excite a strong resonance condition in metalnanoparticles, which can lead to enhanced, localized electromagneticfields.

SUMMARY

A nanostructure is disclosed which includes a photocatalyticnanoparticle, a first tier of metal nanoparticles, and a second tier ofmetal nanoparticles. Each metal nanoparticle of the first tier is linkedabout the photocatalytic nanoparticle, and each metal nanoparticle ofthe second tier is linked to one of the metal nanoparticles of the firsttier and is located a distance from the photocatalytic nanoparticle thatis greater than a distance between a metal nanoparticle of the firsttier and the photocatalytic nanoparticle.

An exemplary method is also disclosed for assembling an exemplarynanostructure. The method includes attaching a first linker about aphotocatalytic nanoparticle, attaching a second linker to a first metalnanoparticle, and attaching the first metal nanoparticle about thephotocatalytic nanoparticle by connecting the first linker and thesecond linker. A second metal nanoparticle is attached to the firstmetal nanoparticle by attaching a third linker to the first metalnanoparticle, attaching a fourth linker to the second metal nanoparticleand connecting the third linker and the fourth linker.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are included to provide a furtherunderstanding and are incorporated in and constitute a part of thisspecification, illustrate embodiments of the invention that togetherwith the description serve to explain the principles of the invention.In the drawings:

FIG. 1 is a diagram of an exemplary nanostructure having two tiers ofmetal nanoparticles.

FIG. 2 shows graphs depicting exemplary intensity enhancement and metalnon-radiative lifetimes in relation to wavelength for the differences inenhancement between single tier and multi-tier nanostructures.

FIG. 3 shows graphs depicting exemplary effects of different types ofdielectrics on a resonance wavelength of a nanostructure.

FIGS. 4A and 4B are high level diagrams showing an exemplary method ofconnecting a photocatalytic nanoparticle and a metal nanoparticle.

DETAILED DESCRIPTION

FIG. 1 is a diagram of an exemplary nanostructure 100, including aphotocatalytic nanoparticle 110, first tier 120 of metal nanoparticle,and a second tier 130 of metal nanoparticles. Each metal nanoparticle120 a to 120 f of the first tier is linked about the photocatalyticnanoparticle 110 (e.g., attached to the photocatalytic nanoparticle orabout a void created to represent the photocatalytic nanoparticle), andeach metal nanoparticle 130 a to 130 f of the second tier is linked toone of the metal nanoparticles 120 a to 120 f of the first tier,respectively. The metal nanoparticles of the second tier 130 are locateda further distance from the photocatalytic nanoparticle 110 than adistance between the metal nanoparticles of the first tier 120 and thephotocatalytic nanoparticle 110.

Embodiments of a nanostructure including first and second metalnanoparticle tiers can confine light approaching the nanostructure inalmost any direction and excite surface plasmons. These surfaceplasmons, in turn, can produce a focused electric field in a resonancecavity comprising the first tier 120 and second tier 130.

The two tiers of metal nanoparticles 120, 130 shown in FIG. 1 can act asa three-dimensional feedback structure or resonance cavity thatamplifies the electric field within the resonance cavity viaelectromagnetic enhancement. The enhanced field can be highly localizedat a place serving as a “hot spot” for the field enhancement and a sitefor locating the photocatalytic nanoparticle 110. These metalnanoparticle assemblies can be, for example, any kind of metalnanoparticle structure assembly that has a dielectric constant having anegative real part. Examples of metals for the metal nanoparticlesinclude gold, silver, aluminum, copper, titanium, chromium, and othermetals capable of supporting surface plasmons.

The photocatalytic nanoparticle 110 is a material having the capabilityof producing excitons, such as a semiconductor material, or any otherdesired particle to be exposed to an enhanced field. For example,photocatalytic nanoparticle 110 may be a light-emitting semiconductormaterial, such as II-VI or III-V semiconductor material, for exampleCdSe, GaAs, InSb, or LiNbO₃, or other types of semiconductor materials.In other embodiments, the nanoparticle to which the first tier 120 ofmetal nanoparticles 120 a to 120 f is attached can be a magneticparticle, or a place-holder nanoparticle that does not need to interactwith the surrounding feedback structure and which can be removed ifdesired. This placeholder nanoparticle can be on the order of 1 nm orsmaller, for example although the placeholder nanoparticle also may belarger than 1 nm.

The photocatalytic nanoparticle 110 can also be a core semiconductornanoparticle covered with a shell layer of another wider bandgapsemiconductor material to form a core-shell heterostructure. Forexample, a CdSe semiconductor nanoparticle can serve as the core and aZnS epitaxial layer can function as the shell in an exemplaryheterostructure embodiment, although other semiconductor materials maybe used to form the heterostructure. A heterostructure can minimize thenumber of surface traps, yield greater charge recombination (quantumyield), reduce leakage current, and improve injection characteristics.

By varying the number of tiers of metal nanoparticles, the relativesizes of the metal nanoparticles, and/or the distances between the metalnanoparticles, parameter values can be determined that allow forextraction of a maximum enhancement from a given nanoparticleconfiguration. For example, an exemplary nanostructure embodiment mayinclude a metal nanoparticle and/or photocatalytic nanoparticle shapechosen to tune the structure to a particular resonant wavelength.Although the metal nanoparticles shown in FIG. 1 are depicted asspheres, the shape of a metal nanoparticle may be a rod, triangle,plate, pentagon, ellipsoid, or any other desired shape. Other parametersthat may be controlled to increase electromagnetic field enhancementinclude varying the size of a metal nanoparticle and varying the size ofmetal nanoparticles from one tier group relative to another.

Keeping the distances between the nanoparticles proportional whileincreasing only the sizes of the nanoparticles can yield a high resultgain increase that follows a power law with the sizes of thenanoparticles. The non-radiative decay rate does not necessarilyincrease correspondingly. By bringing the nanoparticles closer togetherfor a given nanoparticle diameter set, the enhancement and non-radiativelifetime can, for example, increase exponentially.

The associated wavelength of a photocatalytic nanoparticle, for example,a semiconductor nanoparticle, may be adjusted by, for example,controlling a size (e.g., diameter) or shape of the nanoparticle to tuneit to a desired emission or lasing wavelength. For example, a range ofwavelengths from 490-620 nm, or lesser or greater, can be achieved forCdSe semiconductor nanocrystals by appropriately varying the diameter ofthe CdSe nanoparticle. Wider or narrower ranges of wavelengths can beachieved by using other semiconductor material compositions.

Referring to FIG. 1, the metal nanoparticles 120 a to 120 f and 130 a to130 f, and the photocatalytic nanoparticle 110 can be linked usingorganic linkers. A first linker 111, a second linker 112, and aconnection point 113 between the first and second linkers, is providedbetween the photocatalytic nanoparticle 110 and each metal nanoparticleof tier 120. Third linker 121 and fourth linker 122 with an intermediateconnection point 123 are provided between each metal nanoparticle of thefirst tier 120 and each metal nanoparticle of the second tier 130. Thelinkers connecting the photocatalytic nanoparticle 110 and the tiers ofmetal nanoparticles 120, 130 can be any organic ligand type, such asalkane linkers or polyethylene glycol (PEG) linkers, or another type oflinker composed of organic material.

While FIG. 1 shows nanostructure 100 as having metal nanoparticles 120 ato 120 f attached to six “sides” of a photocatalytic nanoparticle 110,with three orthogonal axes, other embodiments may be arranged on feweror more axes and utilize a fewer or greater number of first and secondtier nanoparticles per nanostructure to focus the field. The pattern ofthe overall nanostructures may be self-similar or any other type ofpattern. Furthermore, some embodiments of nanostructure geometries mayinclude more than two tiers to provide large localized fieldenhancements to amplify (e.g., increase or decrease (i.e., attenuate))the electric field within the cavity. A distance d1 between a metalnanoparticle in tier 120 and the photocatalytic nanoparticle 110, and adistance d2 between a metal nanoparticle in tier 120 and a metalnanoparticle of tier 130 may be set to particular values to controlfield enhancement. Because the nanostructure 100 can be assembledsmaller than the wavelength of light, for example, in sizes on the orderof about 100-200 nm, it can have a very high packing density thatpermits its use in a variety of optical and/or other applications.

FIG. 2 is a graph showing increased enhancement provided by amulti-tiered structure. As shown in FIG. 2, the graph 210 representsenhancement provided by a nanostructure having a first tier includingtwo metal nanoparticles around a centrally located nanoparticle, and thegraph 220 represents enhancement provided by adding a second tier ofmetal nanoparticles. The second tier provides two additional metalnanoparticles, resulting in a nanostructure with a total of four metalnanoparticles around a centrally located nanoparticle. As can be seen inFIG. 2, a nanostructure having only a single metal nanoparticle tier canprovide a slight enhancement to the field. However, providing additionalmetal nanoparticle tiers can substantially increase enhancement. Forexample, providing a second metal nanoparticle tier located a distancefurther from the first metal nanoparticle tier can increase enhancementby up to an order of magnitude, or even greater.

Graph 230 of FIG. 2 represents a metal non-radiative lifetime (i.e., arate at which the photocatalytic nanoparticle couples to non-radiativemodes of the metal nanoparticles), resulting from both the one-tierstructure corresponding to graph 210 and the two-tier structurecorresponding to graph 220. A negligible increase in the metalnon-radiative lifetime is incurred when a second tier is provided to thenanostructure, and the graph of the metal non-radiative lifetime for theone-tier structure coincides (overlaps) with the graph of the metalnon-radiative lifetime for the two-tier structure, as shown in graph230. As can be seen from FIG. 2, enhancement can be an order ofmagnitude greater when nanoparticles are provided further away from thephotocatalytic nanoparticle with no substantial increase of coupling tothe non-radiative lifetime. A significant increase in enhancement can beobtained almost independently of non-radiative loss, by optimizing thegeometry of the system.

The plasmonic resonance condition can change with the dielectricconstant of the environment surrounding the nanostructures. Forinstance, it is possible to tune the enhancement and/or the resonancewavelength of the nanostructure by changing the surrounding coatingmaterial and its corresponding dielectric. For example, an enhancementcontrol variable may be the inclusion of a coating on the metalnanoparticles that has a different dielectric constant than the metal,for example, silicon dioxide. The nanostructures may be embedded in apolymer or other type of material to provide support to thenanostructure. This can permit another aspect of dielectric tuning. Thefrequency and/or intensity of the plasmonic resonance are known to besensitive to the dielectric properties of the surrounding medium. Forexample, the plasmonic resonance can be sensitive to the refractiveindex of matter close to the nanostructure surface.

In FIG. 3, exemplary effects of a dielectric constant on the fieldenhancement of the nanostructure are shown, as well as the correspondingmetal non-radiative lifetime. Graphs 310, 320, and 330 show intensityenhancements in the structure for the surrounding environments of air,glass, and water, respectively. As shown, when the dielectric constantof a material surrounding the nanostructure increases, the fieldenhancement correspondingly increases. The wavelength is alsored-shifted as the dielectric constant of the surrounding materialincreases.

Additionally graphs 311, 321, and 331 of FIG. 3 represent the metalnon-radiative lifetime corresponding to the non-radiative recombinationrate for air, glass, and water, respectively. The non-radiative lifetimeis shown as decreasing as the dielectric constant of the surroundingmaterial increases. FIG. 3 shows that as the enhancement increases by anorder of magnitude between air and glass, the non-radiative decay ratedecreases only by a factor of 2. As enhancement grows, thephotocatalytic nanoparticle non-radiative recombination rate into themetal grows as well, but much less than the actual enhancement, makingdielectric shifting an efficient enhancement tuning method. Not onlydoes the surrounding material provide a means for fine-tuning theenhancement strength and peak wave length, it can also provide supportfor the nanostructure.

Embodiments can include a nanoparticle having non-linear opticalproperties placed in a location of the focused high field (i.e., anenhanced field part of a nanostructure). Such nanoparticle materials mayexhibit second or third order non-linearity. The optical properties ofthese non-linear nanoparticle materials can change as a function of afield in which the nanoparticle is placed. For example, use ofnon-linear materials for the nanoparticles in these structures wouldpermit optic behavior that would normally occur at a high intensity tooccur at lower (e.g., order of magnitude lower) intensity. This cansubstantially increase the potential applications for such a structure,such as more efficient optical switching

A self-assembly method for creating this nanostructure can permitcontrol to be retained over the number of layers, particle materialcomposition, size, shape and/or overall development of the structure.This method can permit the structure to have a self-guided assembly thatyields a high volume of product due to the selective nature ofself-assembly chemistry (i.e. because each particle can selectively bindto another kind of particle, there can be a high yield of the desiredproduct). This can be carried out by using organic linkers that canattach to another kind of linkers but not to themselves.

The first linkers that are attached about and/or to the photocatalyticnanoparticle are linkers that can favorably bond to a photocatalyticparticle at one end and have a terminal functional group at the otherend that favorably bonds to a second kind of linkers. The second kind oflinkers can be selected to favorably bond to the metal nanoparticles atone end and include a functional group at the other end that willfavorably bond to the first kind of linkers. These linkers can actanalogously to magnets. Just as magnets have two distinct poles whichonly connect north to south, these linkers attach only at certain ends,and not others, and use these attachment points to connect together in amanner similar to stacking blocks.

In general, exemplary linkers will be of the form H₂N—R—COOH, where R isan organic linker of any desired length. The H₂N (amine functionalgroup) and the COOH (carboxylic acid functional group) are two exemplaryterminal groups for the different types of linkers. This can permit thelinkers to connect by peptide bonding (i.e., the bonding mechanism ofamino acids), where the amine functional groups can, for example, bondto the carboxylic acid functional groups. In an exemplary embodiment, toform the first tier of metal nanoparticles around a photocatalyticnanoparticle, the photocatalytic particle can be coated with a pluralityof first linkers having one of the two exemplary terminal groups. Aplurality of first metal nanoparticles can be coated with a plurality ofsecond linkers having the other of the two exemplary terminal groups.When these two types of nanoparticles are combined, the amine end ofeach of the linkers can combine with the carboxylic acid end of each ofthe linkers. This permits the nanoparticles to be linked in a way thatcan prevent the first tier metal nanoparticles from attaching to eachother. To add an additional tier of metal nanoparticles, the first tierof metal nanoparticles is coated with a plurality of third linkerssimilar to or the same as the first linkers and a second tier of metalnanoparticles is coated with a plurality of fourth linkers similar to orthe same as the second linkers. These linkers can be connected in themanner described above. In some embodiments, the nanostructure possessestwo tiers; however, additional tiers of metal nanoparticles are possibleand can be added in a manner similar to the first tier or in some othermanner.

FIGS. 4A and 4B are diagrams showing an exemplary method for assembling(i.e., fabricating) a nanostructure. In FIG. 4A, photocatalyticnanoparticle 410 and metal nanoparticle 420 are shown. Thephotocatalytic nanoparticle has an organic linker 401 of any desiredlength attached to the nanoparticle, and the metal nanoparticle has asimilar organic linker 402 of any desired length attached to thenanoparticle. Each of these linkers has a preferred terminal groupattached to the other end.

For example, in FIG. 4A, a carboxylic acid functional group 403 isattached to the terminating end of the linker 401 connected to thephotocatalytic nanoparticle and an amine functional group 404 isattached to the terminating end of the linker 402 connected to the metalnanoparticle.

In FIG. 4B, the amine and carboxylic acid functional groups of FIG. 4Ahave reformed to arrange the nitrogen of the amine group and the doublybonded oxygen of the carboxylic acid group into group 405, effectivelyjoining photocatalytic nanoparticle 410 and metal nanoparticle 420. Inexemplary embodiments, the functional groups used for linkingnanoparticles are amines and carboxylic acids. However, other suitablefunctional groups or chemical moieties can be used for connecteddifferent kinds of nanoparticles.

Because of the unique optical properties that can result from theinteractions between the photocatalytic nanoparticle and the feedbackstructure, and the three-dimensional confinement these nanostructureassemblies are able to achieve, there is great potential for novelapplications for these structures. Unlike a two-dimensional surfacewhere a field can involve a certain polarization, where light entry canbe limited to a certain direction, and where the amount of field insidecan depend strongly on the direction (e.g., nanorods and nanowires thatconfine light only in two dimensions), the three-dimensional (3-D)aspect disclosed herein is substantially directionally independent. The3-D structures described herein can confine light propagating fromalmost any direction, resulting in a capacity for a greatly enhancedlocalized electric field.

For example, the enhanced electric field created by a super-structurearranged around some light-emitting nanoparticle, such as a quantum dotor other photocatalytic nanoparticle, can be used to stimulate anincrease in emission from that light-emitting nanoparticle. The 3-Dsuper-structure can alternatively be arranged around a non-linearmaterial, a magnetic material or even a molecule of heavy water, for thepurpose of confining light and focusing the electromagnetic energy fromall directions into localized spot. Additionally, while 3-D confinementis present in certain existing applications such as photonic band gapcrystals, these crystals can include many defects, and their growth andresulting form can be difficult to control. To generate enhancement, thecrystals are thousands of layers thick. As described herein, a means ofgenerating 3-D confinement can be achieved using several layers ofnanoparticles.

It will be apparent to those skilled in the art that various changes andmodifications can be made in the method and system for accumulating andpresenting device capability information of the present inventionwithout departing from the spirit and scope thereof. Thus, it isintended that the invention cover the modifications of this inventionprovided they come within the scope of the appended claims and theirequivalents.

1. A nanostructure comprising: a photocatalytic nanoparticle; a firsttier of metal nanoparticles, each metal nanoparticle of the first tierbeing linked about the photocatalytic nanoparticle; and a second tier ofmetal nanoparticles, each metal nanoparticle of the second tier beinglinked to one of the metal nanoparticles of the first tier and located adistance from the photocatalytic nanoparticle greater than a distancebetween a metal nanoparticle of the first tier and the photocatalyticnanoparticle.
 2. The nanostructure according to claim 1, comprising: atleast one first linker attached to the photocatalytic nanoparticle; asecond linker attached to each metal nanoparticle of the first tier; athird linker attached to each metal nanoparticle of the first tier; anda fourth linker attached to second metal nanoparticles of the secondtier, wherein like ones of the first through fourth linkers do notattach to one another.
 3. The nanostructure according to claim 1,wherein the photocatalytic nanoparticle is semiconductor material. 4.The nanostructure according to claim 3, wherein the photocatalyticnanoparticle is a quantum dot.
 5. The nanostructure according to claim3, wherein the semiconductor material comprises a heterojunction.
 6. Thenanostructure according to claim 1, wherein the metal nanoparticles areone of gold, silver, aluminum, copper, titanium, and chromium.
 7. Thenanostructure according to claim 1, wherein the nanostructure is locatedin a dielectric medium.
 8. The nanostructure according claim 7, whereinthe medium shifts the resonant wavelength of the nanostructure.
 9. Thenanostructure according to claim 7, wherein the dielectric constant ofthe dielectric medium is adjustable.
 10. The nanostructure according toclaim 1, wherein metal nanoparticles of the first and second tiers arecoated with a material having a dielectric constant different from adielectric constant of the metal nanoparticles.
 11. The nanostructureaccording to claim 2 where the first through fourth linkers are alkanelinkers.
 12. The nanostructure according to claim 2 where the firstthrough fourth linkers are polyethylene glycol (PEG) linkers.
 13. Thenanostructure according to claim 1, wherein the size of metalnanoparticles of the first group is different from the size of the metalnanoparticles of the second tier.
 14. The nanostructure according toclaim 13, wherein metal nanoparticles of the second tier are larger thanmetal nanoparticles of the first tier.
 15. A method for assembling ananostructure, comprising: attaching a first linker to a photocatalyticnanoparticle; attaching a second linker about a first metalnanoparticle; attaching the first metal nanoparticle about thephotocatalytic nanoparticle by connecting the first linker and thesecond linker; attaching a third linker to the first metal nanoparticle;attaching a fourth linker to a second metal nanoparticle; and attachingthe second metal nanoparticle to the first metal nanoparticle byconnecting the third linker and the fourth linker.
 16. The methodaccording to claim 15, wherein like ones of the first through fourthlinkers do not attach to one another.
 17. The method according to claim15, wherein the metal nanoparticles are one of gold, silver, aluminum,copper, titanium, and chromium.
 18. The method according to claim 15,wherein the first, second, third, and fourth linkers are each of a samelength.
 19. The method according to claim 15, wherein the first andthird linkers have one end terminating in a carboxylic acid group. 20.The method according to claim 15, wherein the second and fourth linkershave one end terminating in an amine group.